A known method for measuring the gas concentration is the so-called transmission method, cf. U.S. Pat. No. 3,562,524. This method is based on a measurement of how much the light is alternated at the passage of a gas cell, the difference in light intensity measured with and without the desired gas in the cell indicating the gas concentration. The reliability of the measurement is consequently rather low at low gas concentrations.
The above draw-back is avoided by the photoacoustic measuring method because the signal provided by this method is directly proportional to the gas concentration and not--as in the transmission method--proportional to the difference between two almost equal values. This method is therefore particularly suited for measuring low gas concentrations provided the light intensity within the measuring chamber is sufficiently high.
A known photoacoustic measuring method with a high sensitivity is based on the use of a laser as light source, cf. FIG. 1, the latter partly utilizing the great light power emitted e.g. by a carbon dioxide laser and partly the collimated nature of the light emitted. The light source of the laser is, however, not very flexible concerning a variation of the wave length. In addition, a high-energy laser such as a carbon dioxide laser is expensive, heavy, and large and therefore not suited for mass production.
The almost parallel light is a condition by the known photoacoustic measuring method and may, of course, derive from another type of light source such as a thermal light source or a spectral lamp instead of the light source of the laser. In this manner a high flexibility concerning the choice of wave length is obtained for instance by using an optical filter for the selection of the desired wave length interval. Such a light source is furthermore inexpensive, small, and easy and therefore suited for mass production. The intensity of an almost parallel light beam deriving from said sources is, however, very low as it is proportional to sin.sup.2 .theta., where .theta. is the angle of divergence of the light.
If a highly divergent light focussed on the measuring chamber is used a highly increased light intensity is instead obtained. The latter is, however, encumbered with the draw-back that an essential part of the light hits the walls of the measuring chamber and is thereby partially absorbed. As a consequence the measuring signal is partially reduced because only part of the light passes the measuring chamber, and furthermore a strong background signal is caused by the light power absorbed in the wall of the chamber.